1. Field of the Invention
This invention relates generally to nuclear reactors and, more particularly, to a method and apparatus for repairing a riser brace that lends lateral support to a jet pump assembly of a boiling water reactor.
2. Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. RPV 20 has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and is supported by a shroud support structure 26. A downcomer annulus 28 is formed between shroud 24 and sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of RPV 20 and is coupled to a jet pump assembly 34. Jet pump assembly 34 may include a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), and a riser pipe 38. Thermal sleeve 36 is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 is welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe 38. Riser pipe 38 extends between and substantially parallel to shroud 24 and sidewall 30.
A riser brace assembly 40 stabilizes riser pipe 38 within RPV 20. The riser brace assembly 40 may be fabricated of type 304 stainless steel which, after periods of use, is susceptible to cracking at welded joints. The riser brace assembly 40 is fixedly connected between shroud 24 and sidewall 30, and primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, as shown in FIG. 1. Additionally the riser brace assembly 40 is designed to accommodate for differential thermal expansion that results from reactor start-up and heat-up, and flow induced vibration that is incumbent in the reactor water recirculation system (not shown).
FIG. 2 illustrates the riser brace assembly 40 of FIG. 1 in further detail. Riser brace assembly 40 primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, and includes a riser brace block 43 and two riser brace leaves, an upper riser brace leaf 41 and a lower riser brace leaf 42. Leaves 41 and 42 are attached to riser brace block 43 by welds, and riser brace block 43 is welded to a support pad 130 which in turn is affixed to RPV sidewall 30. At the other end, riser brace assembly 40 is connected to a yoke such as brace plate 49, which is typically a ½-inch thick plate that is welded to the riser pipe 38.
In the riser brace assembly 40 of FIG. 2, welds include the weld which attaches brace plate 49 to riser pipe 38, welds attaching riser brace block 43 to support pad 130, and welds attaching leaves 41 and 42 to brace plate 49. These welds are field welds (made on site). The welds connecting riser brace block 43 to upper and lower riser brace leaves 41 and 42 are shop welds (e.g., pre-fabricated in the shop)
Weld failure due to vibration fatigue, and/or weld cracking due to intergranular stress corrosion cracking (IGSCC) could cause one of the welds joining the riser brace assembly 40 to the RPV 20 to fail. Separation of the riser brace 40 near this weld area could adversely impact safety in BWRs. Potentially, should a riser brace assembly 40 break away from RPV 20 (e.g., at RPV sidewall 30), the riser pipe 38 becomes unstabilized, and the jet pump assembly 34 could be adversely affected. If just one jet pump assembly is damaged, a substantial amount of piping must either be replaced or repaired. Since weld repairs in the downcomer annulus 28 area are typically not practical due to inaccessibility, and the potential for excessive radiation exposure to personnel is real, a need exists for a method and apparatus of repairing cracked riser braces involving little or no in-vessel machining.